Laser plasma sound sources as thermally driven pulsating spheres

Pulsating spheres are abstractions in acoustics and wave physics that approximate ideal monopole sources with point-like geometry and omnidirectional emission. Despite their extensive theoretical use, physical implementations approximating the emission of such sources are scarce. Here, we demonstrate that laser plasma sound sources generated by nanosecond laser breakdown in ambient air can be effectively modeled as thermally driven pulsating spheres. Time-resolved shadowgraphy with a two-laser pump–probe system is used to directly image the plasma formation, thermal expansion and shock evolution. It is shown that the linear acoustic emission of these sources is effectively described by the classical pulsating sphere model when the source radius is defined at the nonlinear to linear propagation transition boundary, and an effective sound speed is introduced to represent the averaged wavefront evolution. This phenomenological mapping enables fast prediction of the dominant spectral features of laser-plasma sound sources under the studied experimental conditions. The findings demonstrate that the acoustic emission of the pulsating sphere can be approximated by localized laser energy deposition in gaseous media. The work contributes to the fundamental understanding of laser-plasma sound sources and provides tools for tailoring their acoustic emission for applications in acoustics, sound reproduction, and related optoacoustic technologies.